From your main Island screen -> Click the battle icon. Then the arena Icon (NOT the World Arena Icon) Then the next arena icon

Don’t press but hopefully you will be on a screen that shows “Battle” and “Rival” tabs on the right? At the bottom of this page it shows a “Defense” Tab. Click the “defense” Tab Click the monster you are removing from the Defense (You will need to select at least 1 monster for your defense though). Click the “Complete” Tab